Liquid-phase oxidation catalysed by copper(II) immobilised in a pillared layered double hydroxide

Abstract

A novel type of supported copper(II) catalyst was prepared by the treatment of a Zn–Al layered double hydroxide containing 2,2′-bipyridine-5,5′-dicarboxylate (BDC) anions with a solution of CuCl 2 in ethanol. The Cu loading in the recovered material, designated as LDH-BDC/Cu, was ca. 4%, with an approximate BDC:Cu:Cl molar ratio of 2:1:1. The catalytic performance of LDH-BDC/Cu was examined in the liquid-phase oxidation of styrene, ethylbenzene, cyclohexane and cyclooctane at room temperature, using acetonitrile/water as solvent, and hydrogen peroxide (30%) or tert-butyl hydroperoxide (70%) as terminal oxidant. The main products were benzaldehyde from styrene (up to 14% yield at 24 h), acetophenone from ethylbenzene (up to 8% yield at 24 h), and a mixture of cyclohexanol and cyclohexanone from cyclohexane (up to 5% yield at 24 h). For styrene oxidation using H 2O 2, benzaldehyde was the only observed reaction product (6–11% yield at 24 h), whereas the same reaction carried out in homogeneous phase using the complex [Cu(bipy)Cl 2] (bipy = 2,2′-bipyridine) gave the byproducts 2-hydroxy-acetophenone (HAP) and benzoic acid (BAc) in addition to the main product benzaldehyde (60–90% selectivity, 9–11% yield at 24 h). When t-BuOOH was used as the oxidant for styrene oxidation in the presence of LDH-BDC/Cu, the byproducts HAP, BAc and acetophenone (each with yields of 1–9% at 24 h) were formed in addition to benzaldehyde (5–14% yield at 24 h). Recycling tests for the latter system showed that the solid LDH-BDC/Cu could be recovered at the end of a catalytic run and used in a second run. For the oxidation of styrene and ethylbenzene using H 2O 2 or t-BuOOH as oxidants, titration experiments showed that LDH-BDC/Cu was significantly more efficient than [Cu(bipy)Cl 2] in terms of the “productive” consumption of the oxidant.